1. Field of the Invention
The present invention relates to an object detecting system in an automatic focusing camera having a macro-photographing function.
2. Description of Related Art
There is known an automatic focusing camera in which a zoom photographing optical system is automatically moved to a focus point in accordance with distance data which are detected by an object distance measuring device based on a triangulation measuring principle. The zoom photographing optical system is also moved, at least partially, by a predetermined amount in the macro-photographing mode (i.e., for photographing objects at a close distance).
For instance, FIG. 7 schematically shows a simple optical arrangement of a known two-group zoom lens which forms the zoom photographing optical system. In the arrangement shown in FIG. 7, the object distance U between the focal point F of the entire two-group zoom lens and an object to be photographed is given by the following equation: EQU U=f.sub.1 (2+X/f.sub.1 +f.sub.1 /X)+HH+.DELTA. (1)
wherein
X: feed displacement of the zoom photographing lens PA1 f.sub.1 : focal length of the first lens group 1 PA1 HH: principal point distance of the first lens group 1 PA1 .DELTA.: distance between the focal point F.sub.1 of the first lens group 1 and the focal point F PA1 L: base length between the light emitting lens 5 and the light receiving lens 6 PA1 f: focal length of the light receiving lens 6 PA1 d: distance between the film plane 7 and the focal plane of the light receiving lens 6
From the equation (1), the displacement can be expressed as: ##EQU1##
Note that in FIG. 7, the second lens group is designated 2, and H and H' designate principal points of the first lens group 1.
FIG. 8 shows a conventional object distance measuring optical system based on the triangulation measuring principle, in which 3 designates a light source, 4 a position detecting element, such as a PSD (photo sensitive detector), 5 a light projecting lens, and 6 a light receiving lens. In this measuring optical system, the light emitted from the light source 3 is reflected by the object, so that the reflected light, which is the distance measuring light, is received by the position detecting element 4 to detect the object distance. The relationship between the distance U of the object from a film plane 7 and a deviation t of the light on the position detecting element 4 is given by the following equation: EQU t=L.multidot.f/(U-f-d) (3)
wherein
Note that a reference position in which the deviation t is zero (t=0) is a point on the position detecting element on which an image of the light source is focused at an infinite object distance (.infin.).
The deviation t can be detected by a value of the electrical current to which the amount of light received by the position detecting element 4 is converted, as is well known. The zoom photographing optical system is moved to the focal point in accordance with the value of the electrical current, on the basis of the above-mentioned equations (2) and (3) to effect automatic focusing. A drive mechanism for such a zoom photographing optical system in an automatic focusing camera is well known.
In the automatic focusing camera, it is necessary to shift the range of measurement of the object distance toward a close object distance side to enable the macrophotographing function.
In the close photographing function, at least a part of the zoom photographing optical system is moved further toward the object from an extreme focal length in the normal photographing mode, so that the focusing operation can be effected, as is well known. In the zoom photographing optical system shown in FIG. 7, the first lens group 1 of the photographing lens system is moved by a constant displacement independently of the displacement which is caused by the automatic focusing device.
FIG. 9 shows a known optical arrangement in which the distance range which can be measured by the distance measuring device is shifted toward the close distance side. As shown in FIG. 9, a prism 8 having an apex angle of .theta. and a mask (not shown) are retractably located in front of the light receiving lens 6, so that the measurable distance range can be shifted toward the close distance side.
Supposing that the refractive index of the prism 8 is n, the deviation t.sub.1 of the image of the light source on the position detecting element 2 in connection with the object distance U.sub.1 can be obtained by the following steps.
The mask mentioned above is provided on the front face of the prism 8 and has an aperture center coaxial to the optical axis l.sub.1.
First, the incident angle .alpha. of the light P.sub.1 upon the surface S.sub.1 of the prism 8 adjacent to the object is determined by the following equation: EQU .alpha.=tan.sup.-1 {L/(U.sub.1 -f-d)}.theta. (4)
The refraction angle .beta. of the light P.sub.1 incident upon the prism 8 having an apex angle .theta. at an incident angle .alpha.is given by the following equation. EQU .beta.=.alpha.-.theta.+sin.sup.-1 [n.multidot.sin {.theta.-sin.sup.-1 (sin .alpha./n)}] (5)
For .gamma.=.alpha.-.theta.-.beta., the deviation t.sub.1 on the position detecting element 4 with the apex angle .theta. satisfies the following equation: EQU t.sub.1 =f.multidot.tan.gamma. (6)
Accordingly, the deviation t.sub.1 of the image of the light source on the position detecting element 4 can be obtained by the equations (5) and (6).
If the object distance at which the light meeting with the optical axis l.sub.1 of the light receiving lens 6 intersects the optical axis l.sub.2 of the light emitting lens 5 is Umf.sub.1, and if the thickness of the prism 8 is neglected, the following equation is obtained: EQU Umf.sub.1 =L/tan {sin.sup.-1 (n.multidot.sin.theta.)-.theta.}+f+d (7)
For a photographing optical system composed of a two-group zoom lens, Table 1 shows the calculation results in which the focal length f.sub.1 of the first lens group 1 is f.sub.1 =24.68 mm, the principal point distance HH=7.02 mm, the distance .DELTA. between the focal point F.sub.1 of the first lens group and the focal point F of the two-group zoom lens is .DELTA.=30.04 mm, the distance d between the film plane 7 and the focal plane of the light receiving lens 6 is d=6.292 mm, the shift displacement of the first lens group 1 at the close photographing mode is 0.5502 mm, the base length L of the distance measuring device is L=30 mm, the focal length f of the light receiving lens 6 is f=20 mm, the apex angle .theta. of the prism 8 is .theta.=2.826.degree., the refractive index n of the prism is n=1.483 (wavelength=880 nm), the measurable distance range is 0.973 m.about..infin., the number of steps of forward feeding movement is 18 among which the range of 0.973 m.about.6 m is divided into 17 steps. The calculation is directed to the shift of the range of 0.973 m.about.6 m to the range of 0.580 m.about.1.020 m.
In the table, 17-18 designates the transfer point between the 17th step and the 18th step and 0-1 a transfer point between zero and the 1st step. In Table 1, the deviation t was obtained from the equation (3) and the deviation t.sub.1 was obtained from the equations (4), (5), and (6).
TABLE 1 ______________________________________ COMPARISON OF POSITIONS OF IMAGE OF LIGHT SOURCE ON POSITION DETECTING ELEMENT AT DIFFERENT DISTANCES IN NORMAL PHOTOGRAPHING MODE AND CLOSE PHOTOGRAPHING MODE WITH PRIOR ART APPARATUS DIFF. IN STEP STEP NO. U(m) U.sub.1 (m) t(mm) t.sub.1 (mm) t.sub.1 -t(mm) (STEP) ______________________________________ 17-18 6.000 1.020 0.1004 0.1274 0.0270 +0.818 17 5.154 0.996 0.1170 0.1423 0.0253 +0.767 16 4.027 0.951 0.1500 0.1719 0.0219 +0.670 15 3.310 0.911 0.1827 0.2013 0.0186 +0.571 14 2.814 0.875 0.2153 0.2305 0.0153 +0.474 13 2.450 0.841 0.2476 0.2595 0.0120 +0.374 12 2.172 0.810 0.2797 0.2884 0.0087 +0.274 11 1.952 0.782 0.3115 0.3170 0.0055 +0.174 10 1.775 0.756 0.3432 0.3455 0.0023 +0.073 9 1.628 0.732 0.3747 0.3738 -0.0009 -0.029 8 1.504 0.709 0.4059 0.4018 -0.0041 -0.132 7 1.399 0.688 0.4369 0.4298 -0.0072 -0.233 6 1.309 0.668 0.4678 0.4575 -0.0103 -0.337 5 1.230 0.650 0.4984 0.4850 -0.0134 -0.441 4 1.161 0.633 0.5288 0.5124 -0.0165 -0.545 3 1.100 0.616 0.5591 0.5396 -0.0195 -0.650 2 1.045 0.601 0.5891 0.5666 -0.0225 =0.755 1 0.996 0.587 0.6189 0.5934 -0.0255 -0.856 0-1 0.973 0.580 0.6338 0.6068 -0.0270 -0.906 ______________________________________ Umf.sub.1 = 1.283 m
From the results shown in Table 1, it can be understood that the adjustment by the prism 8 using the measuring light Which passes through the aperture of the mask coaxial to the optical axis l.sub.1 causes a deviation of 0.027 mm on the position detecting element 4 at the extremities of the measurable distance range in the close photographing mode. This deviation corresponds to about 1 step in terms of the number of feeding steps of movement. Therefore, if the feeding movement of the photographing optical system is controlled directly in accordance with the output of the position detecting element 4, it is impossible to move the photographing lens to a correct focal point, resulting in it being out of focus. For example, in the macro-photographing mode at U.sub.1 =0.996 m, if the deviation t.sub.1 is 0.1170 mm, the zoom photographing lens can be moved to a correct focal point. However, since the actual deviation t.sub.1 is 0.1423 mm, the zoom photographing lens cannot be moved beyond the 16th step, so that the zoom photographing lens cannot be exactly focused. This is because that, in a measuring optical system using a measuring light which passes through the aperture center of the mask coaxial to the optical axis l.sub.1, it is impossible to largely change the variation of the deviation t.sub.1 of the image of the light source on the position detecting element 4 relative to the object distance U.sub.1.
To solve the problem mentioned above, the assignee of the present application has proposed a focus adjusting device for increasing the precision of the adjustment of the focus at the macro-photographing mode, in PCT Patent Application No. PCT/JP87/00293. In this prior application, a prism which has two total reflection surfaces and a mask are provided to substantially increase the measuring base length L in order to adjust the focus, so that the difference in deviation of the image of the light source on the position detecting element 4 between the normal photographing mode and the macro-photographing mode can be limited to approximately 0.0001 mm.
In the aforementioned application, it was found that complete compensation for the difference in deviation between the normal photographing mode and the macro-photographing mode can be achieved if the rate of deviation t.sub.1 is adjusted by multiplying this rate by 1.1130 (calculated by dividing 0.5334 by 0.4794), which equals the change in t from step 0-1 to step 17-18 divided by the change in t.sub.1 between step 0-1 and step 17-18, since decreases in the deviations t and t.sub.1 between steps 17-18 and 0-1 are 0.5334 mm and 0.4794 mm, respectively. The prism arrangement used in the application provides that compensation.
However, in the prior application, since a prism having two total reflection surfaces is used, there are drawbacks as follows.
First, a strict tolerance for the angular dimension of the prism is necessitated.
Second, in order to ensure a large quantity of light, it is necessary to use a large prism or to use a plurality of prisms, resulting in a large optical system difficult to operate.